Wide range amplifier



' i 3' -o+ SIGNAL 3' I2 PLATE SOURCE 4d T VOLTAGE 16, 1941. A. w. BARBER 2,266,403 I WIDE RANGE AMPLIFIER Filed Jan. 26, lss

"-- SOURCE H 2 GTILlZ/ITIM 1 I2 1 SIGNAL j 17 6 MEANS j; PgnnzEv I 10 i v LTAG S Q 6 T /6 u 0URCE 22 23 b v PLATE 2! SIGNAL 2' 26 VOLTAGE S E 25 3 5 SOURCE I 0' l LATE SIGNAL VOLTAGE SOURCE 3ouace l L .INVENTOR Patented Dec. 16, 1941 UNITED, f, STATES PATENT OFFICE 2,266,403 WIDE RANGE MPLIFIER Alfred w. Barber, Flushing, N. Y. Application January 26, 1939, Serial No. 252,902 5 Claims. (01.1'79-171) My present invention relates to electrical wave amplifiers. In particular it relates to means of and apparatus for amplifying alternating current waves of audio, video or other ranges of frequencies and especially of very low frequencies of the order of one cycle per second or less.

One object of my invention is to provide thermionic vacuum tube apparatus for amplifying alternating or pulsating electrical currents over a wide range of frequencies such as audio or video bands. Another object is to provide improved amplification at the very low frequency end of such bands. Still another object is to provide suitable apparatus for amplifying flat topped waves such as characterize pedestal and block-out signals in television scanning systems. A particular object is to filter out distorting impulses i ordinarily generated in amplifiers of such signals by charging efiects in the usual filter and decoupling condensers.

improve the stability in amplifiers having'high gains at low frequencies and to render them insensitive to disturbing impulses such as sudden changes in power line voltage.

Thermionic vacuum tube amplifiers having flat gain characteristics over a wide range of frequencies present several peculiar problems. One of the greatest problems arises when it is attempted to carry the gain t very low frequencies. When interstage coupling systems are designed to be efficient at very low frequencies it is often found that the amplifier becomes unstabledue to the increasing impedances of power supply decoupling and by-pass condensers as the frequency is decreased. Finally a frequency may be reached at which their efficiency is so low that the amplifier oscillates at a very low frequency which phenomenon is often called motor-boating. Also the voltage drops across these condensers may be amplified thus distorting the wave form of the currents or voltages being amplified.

My present system is novel in that decoupling and stabilization filtering is accomplished efii-.

ciently at very low frequencies by means of resistor networks which maintain their efiiciency to the lowest possible frequency. The possible distorting voltages generated across power supply filter condensers are filtered out by these same resistor networks in such a manner that wave- 1 form distortion at very low frequencies ispractically eliminated. One form of this system. permits cathode bias production without resorting to by-pass condensers and at the same time without cathode circuit degeneration.

These and other modes of operation will be Another particular object is to.

more fullydescribed in the following specification when taken in conjunction with the drawing. In the drawing:

Fig. 1 shows a triode circuit amplifier embodying my invention.

Fig. 2 shows a pentode amplifier circuit embodying my invention.

Fig. 3 shows a modification of the triode amplifier circuit.

Fig. 4 shows a direct coupled amplifier embodying my invention.

Figs..5, Sand '7 show various electrical waveforms useful in explaining the operation of my invention.

A conventional thermionic vacuum tube amplifier with resistance or impedance coupling comprises an interstage coupling system wherein a load resistance or impedance is connected be tween the plate of one tube and a source of plate biasvoltage, a coupling condenser connected between the same plate and the grid of a second or' following tube and a grid resistor connected between the grid and ground. The plate voltage supply usually consists in a source of alternating current, a rectifier and an inductance capacity filter. The amplifier plate voltage may be fed directly from the last shunt (to ground) condenser in this filter or from a condenser with one side grounded which is decoupled from the main filter by means of a series inductance or resist-' ance. At any rate for purposes of filtering, regulation and decoupling the lower end of the plate load resistance or impedance will in general be by-passed to ground by a condenser. If on the grid of the first thermionic-vacuum tube a voltage is impressed of the formshown in Fig. 5, a fiat topped pulse of current is caused t flow in the plate load resistor of the tube. As this current flows. however, current is drawn from the condenserdescribed above and the voltage across the condenser starts to fall. The change in voltage across the condenser thus generated is shown in Fig. 6. Since the sum of these two voltages is impressed on the grid of the following tube, a distorted wave as shown in Fig. 7 is produced. It is thus desirable in order to preserve good fidelity in such an amplifier to reduce or eliminate the variable portion of the by-pass condenser voltage from the voltage passed to the grid of the second tube. I propose to show means whereby this may be accomplished.

In Fig. 1, I have shown a single thermionic vacuum tube amplifier connected between a Source of signal and a Utilization means. The Source of signal may be taken to represent a photo-electric cell, radio or television detector, another amplifier stage or similar device. The grid I of tube 2 may be connected to the Source of signal thru a coupling condenser 3 across grid leak 4 or it may be directly connected depending on what the device is. The cathode 5 of tube 2 is connected to ground G or other point of reference potential thru the bias resistor 6 by-passed by condenser l. The cathode or grid bias may be supplied in other ways well known in the art such as by means of a battery or a point on the power supply voltage divider. Plate 8 of tube 2 is coupled to the Utilization means which may be another amplifier stage, cathode ray tube, synchronizing circuit or similar device by means of coupling condenser 9 although direct coupling may be used to a device having a high direct current resistance component of input impedance such as the grid of a vacuum tube. Cathode 5 may be energized by means of a heater the connection of which is not shown. Plate 8 is energized from a Rectified alternating: current plate voltage supply or other source of steady voltage. Resistor I-il connected between the plate voltage supply and one: sideof conidenser I I may be taken to represent" aidecoupling resistor, a filter resistor or the internal resistance of a battery or rectified power supply. The other side of condenser II is connected to ground G and/or the negative side of the battery or power supply. My invention in so far as Fig. 1' is concerned resides in the method of feeding'andloadingplate 8. by means of the series resistor I2 and the shunt resistor I3. Resistor I2 is connected' between the positive. side of condenser II and plate 8. and resistor I3 is connected between plate 8 and ground G. Thus plate 8'- receives a positive direct currentv bias which depends on the values of resistors I2' and I3, the voltage across condenser II: and the current taken by plate 8; The alternating current load on plate 8 consists in resistors I2 and: 13in parallel. Any alternating or pulsating voltages across condenser II are reduced by a factor 12 13 1: 13 11 13 T19R13 +1312 p is p 12 +R12 1a.-

'1: +R13 before appearing at the. input of the Utilization means where T1: is the alternating currentresistance of the tube 2 from plate 8:: to ground, R13 is. the value of resistance l3i and R12 is the value of resistance I2. Thus if stant at all frequencies, this reduction inrpulsating voltage holds down to zero frequency which is of. great advantage in distortionless. amplification of low frequency, flat-topped voltages; Instability due to line voltage changes or: other causes is greatly reduced since it too:- is due. to fluctuations of the voltage across condenser [:t.

Fig. 2 is similar to Fig. l excepttubeZ- inthis...

case is a tetrode having an additional grid I I which may be a screen grid, space charge grid or suppressor grid. Grid I l is energized from the Plate voltage source thru decoupling resistor I6 and across filter resistor I'I. Since the current to such a grid is usually small compared to the plate current this resistor It may be made large and resistor Il becomes quite effective in removing fluctuations derived from the Plate Voltage supply in the same manner that fiuctuations are removed from plate 8.

Fig. 3 shows two triocle stages cascaded utilizing my resistance filtering in the plate circuits of each stage. Tube 20 includes cathode 2|, grid 22 and plate 23 and receives alternating current voltages from plate 8 of the first tube I thru blocking condenser 9 and across grid resistor I9. Plate 23 is energized from the Plate voltage source thru resistor 25 and across resistors 24 and 25 connected in series as filtering means as described above in connection with Fig. 1. Fig. 3 shows an. additional feature of my invention in the manner of deriving cathode bias for tubes 2 and 20.. Voltage to plate 8 is supplied thru series resistance I2 and across resistances I3 and I8 in series. One end. of resistor I8 is connected to the negative side. of the Plate. voltage source and the other end of resistor I8 is connected to cathode 5' and the lower end of resistor I3. The upper end of resistor I3 is connected to'plate 8 and one end. of plate: voltage-supply filter resistor I2; If grid. I is made more positive, current to plate: 8: increases causing itto become less positive; The. current to cathode 5 also increases causing: it to become more positive which decreases. the net difierence in potential between grid I' and: cathode 5 and is hence degenerative. However, cathode 5 is connected to a point 'on the plate load. resistor I3I8 and the. decreased voltage across this loadv tends to lower the-voltage on: cathode. 5t counteracting the degenerative effect; With. tubes having high plate resistances this mode of operation. provides a cathode bias which produces very little degeneration, is-simple in. construction, and provides equal frequency response. down. to zero frequency. Tube is driven from plate 8 by connecting grid 22 to plate 8-:thru'blockingeondenser Sand across grid resistor I3.- Plate 232 is fed'with plate bias voltage thrurfllter resistor 26 and is. loadedwith resistors 24= and 25 connected. in. series between plate 23 and the: negative side of the Plate voltage source. Cathode 21. is connected acrossresistor 215' atitsjunction with resistor 24'. Tube 20'thus operates in a. manner similar to tube 2. The output of. tube 20. is fed toa Utilization means by connecting, the Utilization means directly to plate 23' and ground or' low side of the system G.. 7

Since the. connections shown in Fig. -3 result in circuits wherein the tube plates may be op-- erat'edi at. relatively low and highly stable direct current average potentials they are particularly suited to direct'coupled amplifiers. Fig. 4 shows a direct coupled amplifier of two stages employing: tubes 2' and Zflfiin cascade. The circuits are similar.- to; those; of. Fig. 3 except that plate 8is directly' and condu'ctively connected to grid 22. Grid; I is also shown. directly connected to the Signal source? and plate 23 is shown directly connected to; the Utilization means. This circuit comprises two stages of amplification without conden'sers: associated; with the signal carry ing circuits. Similarly it. shows a twostage amplifier in which bias, loading and interstage coupling is furnished by capacity free resistors. Due to the fact that grid 22 is at a somewhat more positive average potential than grid resistors 24 and 25 should be larger than the corresponding resistors 13 and 18 the actual increase depending on the characteristics of tubes 2 and 20.

While I have shown and described only a few forms of my invention other forms and modifications may be made within the spirit and scope of the invention as set forth in the appended claims.

I claim:

1. In a signal amplifier circuit the combination of, a thermionic vacuum tube including at least a plate, a grid and a cathode, a. source of plate voltage including a positive point and a negative point, a filter resistor connected between the positive point and said plate, a load resistor connected between said negative point and said plate, and a connection between said cathode and an intermediate point of the last said resistor.

2. In an electrical circuit, the combination of, a thermionic vacuum tube including at least a grid, a plate and a cathode, a two terminal source of direct current, an impedance substantially free of capacity between intermediate points and its end points connected across said two terminals, a connection between said plate and an intermediate point of said impedance, and a connection between said cathode and an additional intermediate point of said impedance.

3. In an electrical Signal amplifier a source of direct current including a positive point and a negative point, a plurality of thermionic vacuum tubes each including at least a, cathode, control grid and plate, a conductive signal circuit between the plate of one of said tubes and the control grid of another of said tubes, independent substantially capacity-free voltage attenuating means connected between said positive point and at least two of said plates, a resistance connected between at least one of said plates and said negative point and a connection between the corresponding cathode and an intermediate point on said resistance.

4. In an electrical signal amplifier, a thermionic vacuum tube including at least a cathode, a grid, and a plate, a ground point, a source of direct current one side of which is connected to said ground point, means for supplying direct current from said source to said plate, a substantially capacity-free impedance independent of said current supplying means connected between said plate and said ground point, and a connection between said cathode and an intermediate point of said impedance.

5. In an electrical signal amplifier, the combination of, at least two thermionic vacuum tubes, means for connecting said tubes as cascade amplifiers including a common ground point, -a source of plate current for said tubes one side of which is connected to said ground, means for supplying current from said source to said plates and for limiting low frequency variations in said current to one of said plates at the other of said plates including independent resistors connected between said source and said plates and independent resistors connected between said plates 

